Communication system employing pulse code modulation



Jan. 16, 1951 I J PIER E 2,538,266

COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION Filed May 10, 1945 8 Sheefos-Sheet l BALANCED M-- MODULATOR l LOCAL OSCILLATOR I ATTENUATOR C III C? F/ G. I

C n l l HOMODYIVE 7 nsrecron 12 POLAR/TY a AMPLITUDE 0571:;To9 I 4 1 I ,0 -.a rmusmrrm v fi $550k MonuLA TED" OH H c0050 OUTPUT 2:

LOCAL l 2 O-SCILXLATOR I I SIGNAL C F/G. 6 Ca V ATTENUA 109 III F768 F/GQ FIG/O HOMODYNE DETECTOR z F/G. 7 PHASE SH/FTER LSE FIG. FIG. /2 F/G. /a GEZEI'RA roR ow m;

znz FILTER nzzzzfi, m P NAL TO M N l/E N TOR J R. PIERCE MMIIZM.

AITORNEY n 951 J. R. PIERCE 2,538,266

COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION Filed May 10, 1945 8 Sheets-Sheet 2 FIG. 3 30/ g 5 4.268 Jun-nu n o ,4 e. 6 2 t7n+2 lNl/ENTOR J. R. PIERCE A TTORNEY 8 Sheets-Sheet 5 J. R. PIERCE FIG. 8

COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION TRANS.

LOCAL 08C M/VENTOR B J. R. PIERCE MZMW ATTORNEY Jan. 16, 1951 Filed May 10, 1945 MICROPHONE Jan. 16, 1951 J. R. PIERCE 2,538,266

comumcmou SYSTEM EMPLOYING PULSE com: MODULATION Filed May 10, 1945 8 Sheets-Sheet 4 INVENTOR J R PIERCE Qua-M 3 ATTORNE Y Jan. IO, 1191 J. R. PIERCE COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION Filed May 10, 1945 8 Sheets-Sheet 5 //v l ENTOR J R PIERCE ATTORNEY la n. H6, 1951 J a c Z,53,6

COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION Filed May 10, 1945 8 Sheets-Sheet 6 RADIO RE CE VER LOCAL OSCILLATOR ATTORNEY Jan. 16, 1951 J. R. PIERCE 2,538,2fi6

COMMUNICATION SYSTEM EMPLOYING PULSE com: MODULATION Filed May 10, 1945 8 Sheets-Sheet 7 l FIG. I?

REC.

LR/T TERM.

INVENTOR J. R. PIERCE ATTORNEY Jan 16, 1951 COMMUNICATION Filed May 10, 11.945

J. R. PIERCE SYSTEM EMPLOYING PULSE CODE MODULATION FIG. /3

8 Sheets-Sheet 8 Ia-WW- lNl ENTOR J. A. PIERCE ATTORNEY Patented Jan. 16, 1951 UNITED STATES PATENT OFFICE COMMUNICATION SYSTEM EMPLOYING PULSE CODE MODULATION John R. Pierce, Millburn, N. J assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application May 10, 1945, Serial No. 592,961

.; This invention relates to communication systems for the transmission of complex wave forms ofthe type encountered in speech, music, sound, mechanical vibrations and picture transmission -bymeans of code groups of a uniform number of signal impulses of a plurality of different types or signaling conditions transmitted at high speed. The object of the present invention is'to provide a communication system capable of transmitting and reproducing with high fidelity a complex wave form over an electrical transmission path in such a manner that the signal-to-noise ratio of the received signal is materially improved, the frequency band width required for the trans -mission of the signals being held at the same time toa minimum.

Another object of this invention is to provide improved and simplified methods and apparatus capable of transmitting and receiving signal impulses Over a noisy channel and deriving therefrom signals having a high signal-to-noise ratio.

More specifically, it is an ,objectof the present invention to provide methods of and circuits and apparatus for transmitting in succession a group of pulses in sequence representative of the amplitude of a complex wave :atsuccessive instants of time.

It is a further object of the present invention to provide improved apparatus for determining the code to be transmitted to represent each ofa large number of different amplitudes without which is a function of the amplitude of the original complex wave at the given instant.

I Another object of this invention is to recom-: bine a succession of such single pulses of varying amplitude in a manner to reconstruct a wave form of substantially the same shape as the wave form to be transmitted.

A feature of the invention relates to a Sam-r .pling apparatus for sampling a complex wave at frequent intervals of time.

Another feature of the present invention re- .lates to methods andapparatus for determining 1 the magnitude of an electrical quantity and trans-- mitting a series of pulses representative of said magnitude.

Another feature of the invention relates to ,methods and apparatus for building'up an elec- :;trical quantity which is simply related to and a;

the use of complicated counting circuits, arrangements and. equipment.

' Still another object of the present invention "is to transform a series of pulses representing the amplitude of a complex wave at a given instant of time into a single pulse having an amplitude 2'7 Claims. (Cl. 1'7843.5)

measure of the amplitude of a sample of a com plex wave at a given instant. This quantity may take on the nature of an attenuation, conveniently expressed in decibels.

Still another feature of this invention relates to methods and means for .building up of this electrical quantity step by step by a multiplicative or a divisive process i. e., on a non-linear basis to a total which is a function of to the amplitude of a complex wave.

Another feature of the invention relates to the use of attenuation to be so built up by an additive or subtractive process on a decibel basis to the total requiredthus obtaining the equivalent of acompression of the signal. v

Another feature relates to the transmission of information, in the form of a code, regarding each attenuation introducedor withdrawn.

Still another feature relates to methods and means for receiving 'such transmitted information, decoding and translating it to finally yield a wave form which reproduces with high fidelity the original complex wave.

Other features of theinvention relate to synchronizing and coordinating the various circuits and equipment at'the transmitting terminal with each other-and with the circuits and equipment of the receiving end so as to secure proper operation of the-entire system. I

Briefly, in accordance with the present invention, equipment is provided for generating a control pulse or a group of pulses of predetermined time relation one with another. pulses are employed to control a code element timing circuit whichcircuit in turn generates a series of very short pulses some of which are positive and some negative and some a combination of the two. i

, Apparatus is also provided for sampling or deriving an electricalouantity which is a function of the amplitude of a'complex wave to be transmitted, this sampling means being under control of th'e'control pulse generator. For each of the control pulses a code element timing circuit generates a series of code element timing pulses and these in combination with the sampling means derive an electrical quantity having a magnitude related to the magnitude of the complex wave at the time of the control pulse.

This electrical quantity, as illustrated herein, takes on the character of an attenuation which may be varied step by step, operating on the complex wave sample and being controlled by a re- ..sidual from the sample after attenuation to determine the next step of variation in the atten s These control ation. More specifically, the electrical quantity is in the nature of a plurality of resistances of difierent magnitudes which may be added to or omitted from the circuit in such combinations as to yield an attenuation related to the magnitude of the complex wave at the time of the control pulse. The amount of attenuation; is. tested. from instant" to instant as a smaller and" smaller change is made in its value to see whether, within a certain reference limit and at each step, it-

is in excess of or is below the, desired-magnitude...

If, for example, the attenuation aftenany change. is such that when operating in connection with the complex wave sample the. attenuation is in excess of the desired value, the. last previous-- change is removed, but if the total attenuation is below the desired value, the-last change is. leftin and the next smaller change is introduced for. trial. In the specific illustration of myin'vention given herewith this plan is followed and if the magnitude of the attenuation-dueto thelast ad dition leaves-a residual of' the.- comp1ex=waveless thanan arbitrary reference level, then. theklast added element of attenuatlonisv removed and; an

onsignal is transmitted If thev addition. still leaves a. residual above the: reference; lev.el=,. then it remains in and information to. thiseifect is passed to the remote pointby-the absence of" a transmitted. signal, i. e.,.an;off signal; the identification of the information: being: specified by the time at. which the informational: pulse is r transmitted. or not transmitted;

This. formv t comparing thesample; with. the

' attenuation is .then carried, onstep; by: step; to as far a point as may; be desired and in. any case to 'an extent, so that; the: granularity of the. signal finally reproduced at. a:.receiyin point will; be

within" the. limits ;of fidelityfientemplated for the system.

While. thus setting? up a. code.of..-on=. andfofi.

pulses completely characterizing theamplitude of eachvsample of the; complex; wave, this codeof pulses is transmitted. Sincei the-,amplitude. of

the: sample is completely-defined by the; code combination of pulses. the receivingequipment .is; required: only; to determine;v in. the first in- ;sta-nce the character: of each pulsei. e.,, whether is;an-,onor an.ofi pulse= Consequentlyas long as the receivedpulses are suiiiciently above .thenoise level to permit their character-to be accurately determined. by the: receiving equip- In addition at the receiving station a. control pulse generator, code; element timing. generator anda system of attenuations to: be. introduced or not introduced, all analogous to those of. the

transmitting station,.are provided- In. addition,

decoding, apparatus. is provided in. which the received pulses are. employed to produceanjelectri- .cal quantity having a. polarity and ma nitude similar to. those of the. complex wave, sample. of the transmission end. of. the system. and. thus the complex wave is. reconstructed from a, succession of such reproduced wave. samples...

The invention itself; both as to it'sorganization, and method of operationtogether with other objects and features thereof will. be. better understood from the. following description taken with the. accompanying drawings, in. which:

Figs. 1 and 2.show in. function block, form the various elements and. the manner in which they 4 cooperate to form an exemplary communicatiori system embodying the present invention. Figs. 3 and 4 illustrate the timing and nature of the pulses and waves characteristic of my system. Fig. 5 is explanatory of the method and apparatus used for measuring or determining the amplitude of. a complex wave-sample- Figs. 8 to 13, when positioned as shown in Figs; 6 and '7, give in detail the various circuits and equipment of an exemplary system embodying the present invention; Fig. llrelates to modification of a portionzof-Figs. 9 and 11.

Referring more specifically to Fig. 1 let M be a modulating function representative of any complex wave such as. a speech, telegraph or picture wave, a small portion of which is indicated by curve 381 of Fig. 3. A coder or modulator sends out a series of signals after each of the times tm, tm+1=tm+T, tm+2=tm +2T and so forth. Each series following a pulse=which samples the wave, consistsofi L+-n, signals of.the .o11-off-. variety. (The signals. needenot: be. on; on off buteach; signal distinguishes .betweerrtwo, positions called. on and-Font?" Thesezm-ight:bepositions of. time;.fr,e:- quency or' amplitudes On? might be. trans.- mitted as, a current being. OE and off."a's= that of current. being onzfi' The; first; signal. oil-n the series. or. group. is; useda to;te1l. theapolarity of the function M at the time the: sample.- was taken. The neXII-ln. signals of, thegrounspecify the; ma nitude; of- M at; the time; the sample was. taken with respect to; some ..arbitrary reference; level of voltage or; current; These. it. signals may bev of such. a. character- 31161;.502 combined: (advanta geouslyon abinary system; and so described. in this specification for illustrative purposes). as.to represent alargenumberof different amplitudes. Thus,- for the; case of. eight; signals there may. be sent betweenit and! tm-l-l, the.-ser-ies; of signalscm, on, on, oif-, on,.oiT, on, oil, on000l..0l.01;.. The first zerorcarries the-information; that. M is. of negative-polarity and-the remaining signalsthat M1 is. ofamplitude 1.0.5, decibels-above, reference level, thisvalue being, obtained by binary counting. Herethesmallest element of. change. has beentaken. as-one-half decibel.

One specific means for obtaining such a series or signals; is illustrated. in.the block. diagram of Fig l.- The, over-all. functioning will be. described firstv and. the possible. nature of theblock units will, be: described. later.

While Elli-31111101310113.descrihedi lll be. carried out without frequency conversion, grounding problems are simplifiedi by first impressing the modulating; function M on-abalanced modulator I- oi.-Eig; 1, together with the-output'of. a local oscillaterILof frequency in. hi h. compared. with the frequencies Occurring in; M; Atthe timetm shown; on. Fig.4 3; a. pulse which will. be referred to. asxani M 131-1158;,f110111231l pulsegenerator -VI is impressed om modulator-I... During;the interval.T betweenrtm. and: timi, when another M;pulse;is impressed: on modulator-I; the. output current In has a frequency f0 and an.amplitude proportional to M 'attm, being plus when M? is plus atrtml and minus when M is minus. at tm. At tm+-1 another sample of the complex wave is taken'an-d for the next interval T the modulator output has. the samefreque'ncy ft and' the' new amplitude. This is-indicated' in- 3*'inwhichthe secondiline 302 shows the potential across a condenser (later to be described) and the third line 3ii3 showsthe oscillationsor frequency f6 associated with the successive M pulsesQ It-should' be noted-that 'if M is negative; as in the last groupshown inline #15 I, this is recorded by a reversal of phase of the modulated current of frequency fo. I

Im is fed to a homodyne detector IV associated with attenuating devices A1 .An. Local oscillator power from II is also supplied to IV. Atthe output of IV there is obtained a voltage or current which increases when M increases, is minus when M is minus and is plus when M is plus, although this output need not be linearly proportional to M. a

The output of IV goes to polarity and amplitude 1 detector V. This serves two purposes. First, after a pulse is applied to modulator I at timand after a resetting pulse, later to be described, puts the attenuation of attenuator III at aminimum;

a pulse is applied to V. If the polarity of Mat he is negative an impulse is sent to the transmitter VII and this, preferably in conjunction with a pulse from VI, causes the transmitter. to send an on signal. If the polarity of M at tm is positive V sends no impulse to transmitterVlI which in turn emits no signal, meaning off. 1 Second, the detector V serves to measure theoutput of the homodyne detector IV and to control the operation of the attenuator III in response thereto, as will be later described in detail. .ii:

The attenuation of the attenuatoris controlled i by a number of resistances R1, R2. Rn. These have on and off conditions corresponding-to the digits of the binary numbers specifying the level of M at in or other sampling time with respect to the arbitrary level. Thus, in the example previously given of 1+n:8 the amplitude =is specified by 7 digits. If one of them is beyond the binary decimal point then turning the vari- ..ous attenuations A oil and on in this case changes the attenuation by the amounts listed below:

A1, 32 db. 5, 2 db. A2, 16 db. A6. 1 db. A3, 8 db. A7, .5 db. A4, 4 db.

These seven attenuations are thus capable of producing attenuations covering a range of 63.5 -decibels in one-half decibel steps. Addition of one such channel roughly doubles the decibel range of attenuation available. The attenuators A1 An are controlled electrically by control circuits C1 Cn. These con- 1 trcl circuits derive input from (a) the pulsjezgen- ,erator VI and (b) the amplitude output of polarity and'amplitude detector V. e

The character and timing of these pulses are shown in Fig. 4' which is an expansion of the time interval between two sampling pulses or the time required for one cycle of operation. Here the second time interval T is shown. The negative ",'pOltlOI1 oi the M pulse resets the modulator I and the positive portion immediately thereafter "takes the sample of the complex wave. At the same time the initial negative pulse shown on each of the 1n of Fig. 4 places all As in the minimum attenuation position. In the meantime the sample of the complex wave operates on the homodyne detector IV.

For instance, at tm+1 a setting pulse place all A's in the minimum attenuation position. Then 1' a polarity pulse P is applied to V and the polarity f of 'M at tm+1 is transmitted. Following this positive pulse is applied to 01. This changes All mediately thereafter a negative pulse is appliied.

above the arbitrary level upon the insertion of attenuation the position of A1 does not change and the attenuation stays in. No pulse is sent to the transmitter VII and no signal is sent, indicating foif! If, however, the amplitude falls below the arbitrary level on the insertion of this attenuation, the position of A1 is changed back by the negative pulse and at the same time a pulse is sent to the transmitter VII. This, perhaps in combination with a pulse from pulser VI, causes VII to send an on signal.

Next, positive and negative pulses are applied in turn to C2 Cn resulting in either the insertion of the steps of attenuation they control and transmission of off signals or in the insertion and subsequent removal of the respective attenuations and transmission of on signals.

After the 1-1-11. Signals specifying the polarity and amplitude at tm+1 have been sent, VI sends a resetting pulse to the controls C1...Cn. This puts all As at the minimum attenuation position.

1 This pulse sendsa marker signal '1 from the transmitter denoting the end of one interval and the beginning of the next. This pulse is simultaneous with the negative pulse applied to modulatorl.

In a review of the system it will be convenient to refer in further detail to Figs. 3 and lshowing the pulses required from the pulse generator VI. At the top there is shown a series of pulses coming over the channel M to the modulator I and a sample 302 of the complex signal wave 301 at the instant of each modulator pulse is taken. Operatingon electrical elements the sample causes the modulator to emit, until pulsed again, a current In of frequency f0 and amplitude proportional to M at the sampling time. All this is indicatedin the second and third lines of Fig. 3. A polarity pulse supplied to polarity and amplitude detector V arrives an interval later and this is succeeded by pulses over channels I, 2 n which operate on the attenuators A1 An. There may also be a pulse channel T to the transmitt'er asshown in Figs. 8, 9 and 10, necessarygif the o signals are transmissions and not merely omissions. All this is shown in 4 which is an expansion on a time basis of one of the sampling periods of the system.

At the time of the M pulse, channels I to n tions. Next the polarity and amplitude detector V is pulsed by the polarity pulse P. This results in a pulse to transmitter VII if the polarity is negative and an on signal from the transmitter, or no pulse to VII if the polarity is positive and an off signal from the transmitter.

The n amplitude channels are pulsed in sequence; first positive and then negative. positive pulse insertsattenuation, the negative pulse removes attenuation if the attenuated output of IV has fallen below the arbitrary level and in doing so sends an on signal, otherwise, the attenuation stays in and there is an off signal.

As shown in Figs. 8, 9 and 10,,the channel T ap- I and puts in say 32 decibels of attenuation. Inlewhether an on, off or a marker signal isto be used. These pulses will give an on signal from the transmitter in combination with a pulse from A1 AnOI' V. 6

While a number of different types of devices may be contrived to perform the functions of the representative blocks of Figs. 1 and 2 a better understanding of the invention will be obtained The T7 tbyispecific illustrations of. circuits zto accomplish tthe idesired :en'ds. These will:.now :be "described 'zbutdt lS ItOFbB understood: that .theyzarezillustraative.onlyandthat.myrinventiomis not' limitednto zilthesspecificacircuit arrangement shown.

lSFor :a .description 'ZOf the transmitting end :of mnyzsystemnreference should bemadeitoi'Eigs. "8;: 9 rand .lOrin lwhich the variousrb'lock':componentsrof are represented byithe-samelftomanznumtbers.

The complex waveitransmittedizis impressed on itheaibalanced'smodulator :system.-I,z.this .complex wave having come from any suitable sourcezsuch aas aimicrophone 28 95 through 'apprQpriateJtermirnaleequipm'ent 866. It is then sampled rperiodi-l deadly rand ;:goes through the ;;process .;to :be idescribed iniurthertdetail.

--Pulse g-enena'tor :It will be advantageousxtosnow-:describe ithe pu-lseagenerating; system V1,. one 'forrn of which is v.zshoWn intdetail in Fig..10. :Th'ea-first controlling --element in this portion ofathe' systemiszarelaxa- :-:tion.oscillator comprising a-gas .tube I-BIG. This .srelaxation: oscillator is ,ofea. form well: known in the art and includes a resistance IGI I foirchargting-acondenser IOI2 from the batterylllllt. :As-

.sumingthat, to start with-the. condenser I41 I 2 .is discharged then .on closure ofuthe. circuit lit -.is

.;charged: at a rate determined -byithe resistance 1 .gpariticular, by the values of .the elements. 'IilI I to *IDM taken with the potential of thegrid of tube IBIS asxdeterminediby the potentiometer IIlI 5.

'While any ofseveral forms of'relaxation circuits .zmay beused' at" this point the; one'shown is sim- ;ple' and satisfactory. Its operation ismorefully described in many, places'suchason page 184- of "*Illtra-"I-Iigh Frequency Technique, by Brain'erd *et' al.;published' by Van Nostrand Company, 1942.

fcontrol the emission of pulsestovariousparts of J the circuit. This is'accomplished'by connecting across the element It I la 'delay'circuit' -I I made ':up of identical sections or *inductance"-and capacitance connected seriatim. A' positive pulse "travelsthrough this network; the time-oi. arrival *at each section being uniformly--spaced and; giv- '=ing rise to corresponding positive and negative pulses "going out -over"channels I,- 2 nifor "vpurposes to be described. The delay network is terminated by a load Ie40of-proper:value to-sup i? "press any refiected wave. i

The parameters of the relaxation 'oscillator "may be adjusted so that pulses,arederivedaacross resistance IIlI4-=at any frequency desired. .For

"the purposes of my inventionitl is preferredzrto '"have a sampling frequency 'higher thanithatiof the highest frequency'component inithezcomplex *wave to'be transmitted. 'Iffiior example'rthis wave is'to be a speech waveand dtis desiredi to transmit all components up to 41000+cy1esthen message means roflthe delay :circuit; or timestick" I 0 I6, one hasavailable-cat thetends of .theirespee- 1tive;sections,:2positivepulses similar to that-lini- Qtiflitfid in I III 4. and spaced in .time,'.-one';afterithe :other, byiani'intervahdetermined byitheeelements in1a:sectionz of :the .timestick. -At time tm when atherpulse is. formed at :IDM. it is transferred im- -mediately to tube 'IflZU'and isthen transferredto "modulator-Lars.ther Mrpulse of Fig. 3. Thefunction of this pulse will be described hereinafter.

;At:':one element: ore-time later the positive; pulse ":from 5 ID I l will. have. :reached .theypoint P v:onuthe :timestick and this'will be identified as theP pulse. ;This?;-P:. p.ulse operates on=the gridot .tubevIIIZI 1.15 to give ":a positive rlpulse cover the resistor I 022 which pulse'ris transmitted .to =.the .polarityaand aamplitudezdetector V, appearing there .as agposi- --;t1ve'::pulse: of-aarform. and at .a time indicateid -by lline?R0f.-Fig.24.

Q-AtitheF en'dzof succeedingxelements of (time-(the. :positive :plllSe :irom iIIH i .will -:arrive at points .I,t2 -..'?n:0f thettimestick andthenareitranstferred'respectively to'the grids of tubes. I 08 I I 082,

N33, etc. Each oithese pulsesin its:subsequent.

:.path-is convertedizinto .a positive-negative pulse,

in a manner andfiorythe .purposehereinaftendezscribed.

iIn addition-ito-these pulses .it is: desired". to; send a; group' oi corresponding-pulses toxthe: transmituter 1VII whicha-pulses'will be identified .aszTIirpulses and are -in'dicated inthebottomline. ofiEigie. .;It will be noted that the first pulse. in.-.this' eyelet-is. longer and of greater amplitude thanthe'asubsequent pulses; this for reasonsiwhich Willaappear 'later. The first T pulse may, for instance;ibeapproximately twice the length ofthe: subsequent pulses. For the formation of the T pulses .a chain of tubes I024 to H129 is provided. The-gri'd-anf tube I024 is operated on directly by the pulse 0 from M. In the cathode circuit there is included the resistance I03I paralleled'by the condenser-I032. -A* positive pulse'is generated across I03I and the duration of this 'pulsewould ordinarily be the same as the duration of the M pulse.

(However, the addition of a.condenser I032" will "lengthen this pulse ..and v.the condenser :isrso .chosenas to approximately double. the duration; The positive pulselfrom Illiii is appliedto the grid of I025,..the. cathode circuit of which includes the resistor. H130. There is then transmitted over. the JI .channel. to the transmitter Vll, atthe1time?of .theJMpulse .ontthe .timestick a positive. ulse 'of approximately twicerthe duration of ;the M.pulse. .JIherelative.magnitude.of this pulse can bacon- ...trolled by.addusting .the resistance I0 I I. in series with the. .timestick I 0 I E. As. the ipositive puise over IBM reaches successively the points JP, I, 2, n'the grids of the tubes I026.to Ifl29are operated tupon and: set .up positive; pulsesv across I I 030 .which, is a. common cathode resistorLfor; all the :tubes I025 .to I029. Consequently thereg-is transmittedthe desired series of -positive.,pulses to the transmitterVII indicated inFig-A. Eorthe purpose ofisynchronizing these pulses with-the remainder ;of-. the equipment,. delay circuits .may be introduced wherever necessary. Onesuchlde- :ilaycircuit risshown in the..T channelat I 05%.

aFollowing-the 2M pulse and the? .pulse ithe 701r-numberof sections in the timestick willusually-be madeequaltotthenumber of digitsrequiredtior ,.--'settingrup..the amplitude :code to .be. transmitted zjromvVII. :IIfLthere shouldbe nof thesethen-the number or, .possible codes by..permutation .of' .fon lliiand fofl?-. signals would be=2. Thus-"if .thesnumber of digits in the code is seven this will make possible 128 combinations, so that in the system it will be possible to discriminate between amplitudes of 128 different values.

Associated with conductor I for the timestick and-tube IE8! is a circuit comprising tube I961 and transformer Hill. The tube 106i is shown as a double triode. The grid of the left-hand section of this tube receives a relatively large positive pulse at in which is then converted at the plate to a negative pulse. This negative pulse is transferred through transformer Hill as a negative pulse to the control of the corresponding attenuation device device Al in Fig. 9 and serves as hereinafter described to set this attenuation element to a minimum attenuation. Shortly thereafter a positive pulse arriving over conductor- I to tube 908i is inverted and appears as a negative pulse on the grid of the right-hand sec- 1 tion of tube i565. However, the load circuit of tube 588i includes the inductance ifiQi which causes the negative pulse generated on the plate of ml to be immediately followed by a positive pulse so that the pulse arriving on the grid of the right-hand section of I06! is a negative-positive pulse. This pulse in turn is inverted by the right-hand section of tube N16! to a positivenegative pulse which is then transmitted through the transformer Hill to the control circuit of attenuator Al. The character and timing of this positive-negative pulse is that indicated in line I of Fig. 4.

A similar circuit is associated with each of the conductors 2, 3 n to give at the time'of the M pulse arelatively large negative pulse to the corresponding attenuation control devices setting each attenuator to a minimum attenuation and at a later time transmitting a positive-negative pulse to the control devices of the respective attenuators in the same manner and at times indicated by lines 2-n of Fig. 4.

Various electrical elements appear in the cir-' cuit associated with each of the tubes thus far described. Thus across the primary of the transformers Hi'li, etc., there appear the resistor r1, T2, and 7's. These are present elsewhere also and they are for thepurpose and of a value to sharpen the pulses which are transmitted by the corresponding transformers and to suppress their dififerentiating action, thus preventing the setting up of an additional reverse pulse. Other elements all serve purposes well known to those skilled in the art and need not be described further.

Modulator I A description will now be given of the modulator circuit-I shown in Fig. 8. A complex wave made up of numerous frequency components, the

highest one of interest being for the present taken as M306 cycles, arrives at the primary of transformer Sit]. At time in; a positive pulse arriving on the grid of tube i629 is inverted by transformer M9 and is, impressed as a negative-positive pulse on the plate and cathode of diodes iii and 852. The negative pulse causes current to flow through 852 and discharges condenser sic.

thereafter it causes current to flow through 8 charging the condenser 8 i i to a definite potential, potential being equal to this positive pulse plus that of the amplitude of the complex wave M at he. minus a constant bias potential determined by battery SIB. The grid of triode 82B is held at this potential minus a biaspotentialdue to .82! until another pulse is applied at tm+1.-The

When the positive pulse arrives immediately 83F) will reverse in phase, still being of an ampliwhich later is used for detecting the c ma and 10 output of the amplifier tube 820 appears as a. voltage across the resistance 825 and controls the operation of a conventional balanced modulator of any suitable form. As here shown it involves two varistors vi and 02.

Associated with this balanced modulator is also a source of local oscillations II of frequency in high compared to,{

thepulse frequencies present in the system. .In

a manner well understood in the art there will appear in the secondary of transformer 83!] a wave f of carrier frequency in, the amplitude of which will be proportional to the potential across resistor 825. The phase of the voltage in the secondary of 830 reverses as the voltage across 825,

The output of the secondary of 830 is connected to the grid circuit of a pentode. 835 which, in turn, yields an output current Im. This current is almost independent of the load con? sisting of a resistors!!!) and an antiresonant cirg ,4

reverses.

cuit 831 is parallel and associated elements to' be described hereinafter. It also gives a proportional voltage Cm across the resistor 900.

Local oscillator II stant frequency.

The load of the pentode 835 includes the plurality of attenuators connected in tandem and shown in Fig. 9;"These attenuators comprise a plurality of tubes 945; 955, etc. connected j-in'r tandem the gain or loss being determined by the amount of controlled attenuation introduced. The pur ose of these attenuators is to receive at the input circuit of the first attenuator a voltage proportional to the current Im and to reducegj this voltage step by step to. as nearly as possiblef a certain reference level, all inafter described.

Homiodyne detector IV purpose of which is to demodulate the arriving current of freouency ft. The homodvne detector in the manner hereif Associated with the last stage of the attenuators is the homodyne detector circuit IV, the

comprises a tube shown as a triode 9!, the purpose of which is to serve as an amplifier but still more to be of a character to add no appreciable j attenuation to that which has been introduced in the load circuit of the last attenuating pentode- 985.- Associated with the outnut of 9|}! through f the transformer 903 is a demodulator circuit comprising two varistors 9M and 9&5. It'is supplied with oscillations from local oscillator II.

The i output of this modulator will give a voltage across j resistor 901 of magnitude dependent on the amplitude of Im as modified by the attenuators. .I

As long as the high frequency current Im is of one phase as represented in the first three sections of the third line of Fig. 3 one terminal (say the right-hand terminal), of resistor 9B! will --be positive and this will be referred to as a positive output. If, however, the magnitude of the complex wave takes on a negative value, as shown by the fourth pulse of line I, Fig. 3, the high frequency wave appearing at the secondary of m litudeoi the complex wave sample.

asss zeey- 1 Poldr-z'ty and am litudeaerator-W :polarity and amplitude. detector V is' shownin. the: lower. box ..of I Fig.,.9'.1 If. the homodyne detectonoutput. over .the.r.esistors .8 B1 and 908' is.

abovasome -arbitrary 1-reference';lvelf'controllable by. bias. SZ'Lcurrent -fiows in' oneor. the other ofiith'e .triodes 924' or 9251. If the ,homodyneout- 'putis in..the directioirto raisethe potential of the gridfil'of; 924 andihas suific'ient amplitudethen the grid '..of..'tube 926 .will be. highly negative. There'- fore.-a. pulse comingfifromttube' i021 and applied inthe .grid'l circuit through transformer 821 will cause no pulse in. the output of tube..926 and no puls'e onthe line leading therefitomtdthe'transmitter VII." If"the.homodyne' output has the opposite polarity; the grid? ofi926 willfibe only:

slightly, negative; and the pulse" through trans former. 921 WiIlZcauseplatecurrentrto now, send:- meta pulse via"transformer;928i-' If'the-outputpfthe"homodyne detector is-larger' than the arbitraryjvalue"in either direction cur.-

rent will flow in'dibd'e"929because of'the drop" over resistor 922 caused. by the flow of current through the respective tube 924 'or'925 thus applyingsa positive bias to the attenuator controls and preventinga negati-repulse comingfromthepulse generator tdany. particular rcontrolfrom. remov ing'the attenuation.

' Attenuator'm.

IIREigi-Q-thBIE. arersh'own; aFphirality; of attenu-v ators iandrzattenuat'or control bircuitszonafor each:

'digiirin; theeampiitudeicode: Three-such unitsaresshowmbut 'inasmucha-as their action is idenetically,"thesamerexcept for-*timing: it -is necessary to; describe only..- oneofthese; identified by the block llll which. includesthe first attenuator of the.series... Each of the. units. III..oompr-ises a.-

grid-vresistorin' that network any degree: of" attenuation may be obtained:

'Eachmf thet'tubes'945; .965;.etc:.w-ill have awgain:

whiclrisdetermined among other thingsby the" impedance-of thecoupiingtnetwork from one :tube:

toothe\next.-. This,-will .be armaximum when no attenuation isintroducedi-andthe over-all: gainf forathe series oftubes,.-unde1'-. this condition-.- will beassumedas definite andwill. take onanypre-- determined ivalueflfromesay 1 ..to'. as. high' 'a. figure aszdesired. When referencezinethis specification islna'deto .attenuatiom it isunderstoodthatrthis w refers;to..the attenuationor. loss indecibels fiiomthat predeterminedgain.

The .attenuationcircuit isessentially a shunt feedback ..amplifier.. It; includes the resistances RiieandcRr' and.connectedimseries; the intermediate. point beingrconnected ..to .the three-stage a mplifierl including the tubes. 94!; 94H. and. 943i withresi'stance.capacitance coupling, there. being, a feedback connection. from. the plate of the .last tub"e1.943. through.. condenser. 944 to. the. grid. of

' 941,3. It will beappreciatedthat someother'form oflcoupling,,.asi-.antiresonant.coupling may be. used. There. is. either. zero or... auvery I large. gain.

.ardundlthe;.1oop, depending err-whetherthe con.-

tiol..voltage. applied..toterminalsu and. .b.- cuts. ofi one or moreztubes in-th'e. loop, or allows themto.

operate. In this instance the loop is "opened blv' a sufiioiently high negative bias on tl1e=grid 'of tube 942; Under-th'ese= conditi'ons the coupling resistance of A1 from tubes 945 to 96515 essen tially R1+R1 and the I attenuation for: this stage is a minimum; If a positive potential ofsuffi cient-value arrives'at the point 5 then the loop is closed and ifthegain through the circuit con-- taining'tubes -94"! to 943 ishigh then R1 is vii tualy 'short circuited andthe coupling resistance of A1 is essentially R1, almost independent of gain. Thus a 'high accuracy of control of A'i can be obtained independent of" tube" characteristics:

Whereas for illustrative, purposes the control has been shown on one tube only it may be desirable" tocontrol the bias'onseveral or all'tubes'in order to"open the loop-completely.

The control'partof the attenuator comprises'a purality of diodes 95L 952, and 953 'and associ-.- ated elements: Wh'enapositive pulse over ch'annel I, supplied through tubes liifltand OBI of" Fig: 10, is applied the diode95l conducts, ch'arg-- ing'the condenser 954; applying apositive poten-- tial to point I) and closing:- the feedbackdoops thusdnoreasing' the attenuation A1; lfr'nmedi'atelythereafter a negative pulse-i's'applied through" transformer 955 todiode 952. If this-is the onlypulse present on diode 952'; that is, if insuflicient positive bias 'arrives' from the amplitude and a larity detector V," meaning a= homodyne detector. output beow a' certain reference, the-negative pulse discharges condenser 954 and opens-the feedback loop thus removing'the 'attenuation' A'i. At thesame time current flowsin' resistance 958 I and through diode 953sending'anegative pulse to the transmitter resulting in the transmissionof an on signal. If there is a-positive bias of suflicient magnitude from the amplitude detector through diode 829, meaning alarge *homodyner output, the negative pulse: through transformer: 955 is not sufiicient to cause current to flowthrough: diode- 952. Consequently: the: attenuattionn'emains in and noipulse'is'sent to the trans mitter:

If the'attenuation A1 remainsin, the-residuali signal arriving at tube 901 is correspondingly. at tenuated. With the arrival ofthe next pulse; corresponding to channel 2; the sample-is againtested by the: introduction of attenuation: A2, followed'by its removal if necessary as determined: by th'e test.

This is made clearenabyreference to Fig; 5. in:- which for simplicity ittis assumed that, exclusive of the polarity pulse, a 7-place amplitude code is to be used on a binarysystem. This makes it possible: to. .discriminatebetween about 128 dif-v ferent amplitudes on a decibel basis; If the-am:v plitudeis .tobe expressed in terms-of areferenoelevel=eo audit the smallest. change to-be observed" isone-haIf decibelthen the range of amplitude: onapower basisv would be from-.-.5 decibel to 63 5- decibels, as suggested on the table of page 8. The first attenuation would introduce a lossof- 32 decibels, the. hex-ta loss of 16 decibels and so. forthandWtheroperation-of the circuit tissuch. as tointroduce a totalattenuation indecibels which isasmearly as possible equal to the decibelampli-- tude. of. em: as compared with the reference -1ev-, el '60:.

If, forrillustlation; the. sample amplitude emappearing'across the resistor '90!) is'46.6 decibels: above. '60 :then. the introduction .of A1,. ,on the -arrivalofpulse overrchanneLl, with anattenua' v tion' of-v 32-decibels Will leave -a residual of 14.6

decibels, as indicatedat a. This -is.,substantial1y,

above reference level and therefore A1 stays in. Had it been less than zero decibels the attenuation would have been removed. The positive pulse over channel 2 now introduces A2, of at-f tenuation value 16 decibels, giving a total attenuation of 48 decibels and a reduction to -l.4 decibels, which is less than reference level as indicated at b and so the negative pulse removes the attenuation. The pulse over channel 3 now introduces A3,. of attenuation value 8 decibels, giving a total of 40 decibels and a reduction of the residual to 6.6 decibels as shown at c, more than reference level and so As stays in.

The next step A4, of attenuation value 4 decibels, gives a total of 44 decibels. The residual is now reduced to 2.6 decibels and therefore A4 stays in. On the next step A5, of 2 decibels value, is added'and brings the total attenuation to 46 decib'els with a residual of .6 decibel and therefore A5 remains in. As introduces an additional 1 decibel giving a total of 47 decibels and a residual of- .4 decibel, less than reference level and therefore As is removed. Av, which uses up the last step in the 7-digit code assumed, introduces .5 decibel for a total of 46.5 decibels. This leaves a residual of +.1 decibel and so A7 remains in. Thus the total attenuation introduced in front of and a corresponding code of'off, on, off, off, off, on, off '(or the equivalent of will have been set up. The marginal operation of the circuits is so adjusted that since the amplitude is in excess of 46.5 decibels, thenegative pulse for A7 would not be sufficient to'remove' it.

If slightly less than 46.5 decibels, the negative pulse would have removed it.

It will appear from the above explanation that the final total attenuation introduced is propor-' tional to the amplitude of the sample, within the smallest adopted decibel step as applied to the reference level unit. The percentage accuracy will be independent of the amplitude of the sample. This maybe shown as follows:

Let

residual to slightly below 0 decibel level, this corresponds to an input voltage e2, where Therefore maximum error for em=6 per cent.- This is the same for large or small input amplitudes within range of ea to r On completion of this process the voltage de l livered to the input of tube 98 i has been adjusted as"nearly as possible to the constant arbitrary reference voltage eo. Since the total attenuation At is given by the binary number represented by n "ofi or on signals, this number also expresses the amplitude of current Im and hence the ampli- 1 tude-of M at the sampling time.

By using a code of fewer digits the system would be simplified but at the expense of range or granularity. By extending to a code of a higher number of digits any degree of fineness of -granularity and corresponding fidelity may be 1 obtained.

It will be observed that this mode of building up attenuation to match a sample amplitude is essentially a multiplicative or divisive process starting with. large decibel steps and going to decibel basis. characteristic is peculiar to this system and con stitutes a distinguishing feature, as is pointed out in the appended claims.

Transmitter unit VII During this procedure there has been arriving at transformer 850 a series of pulses over chan--- nel T, one for each pulse from the pulse generator, timed as indicated on the bottom line of Fig. 4. These pulses may be used to operate on; a grid of the tube 855. In addition, there arrives at the transformer 852 certain pulses, one for each on pulse, relating to'polarity or indicating that one of the attenuators has been introduced and then removed. No pulse will come to the transformer 852 if an attenuator has been intro--- duced but not removed, this corresponding to an; oil signal.

The secondary of the transformer 852 operates on a second grid of tube 855 and this tubein turn controls the transmission or absence of transmission over a suitable medium to a remote station. In Fig. 8 the tube is shown as controlling a transmitting terminal unit 860 for.

a radio'channel on a suitable carrier but it is to be understood that the pulses coming from the tube 855 may go directly to any suitable trans-- mission path such as a pair of wires, a coaxial cable, etc. In such cases it i not necessary and may notbe desirable to use the pulses for modulating a carrier. The connections of the transformers f. and 852 are such that a pulse arriving at 85!! alone will not cause the transmission of asignal but the simultaneous presence of a pulse on 850 and on 352 would be effective in causing such transmission andwould correspond to an on signal. This is so except in the case of the first broad pulse of the T series which is purposely made greater in amplitude than the others so that it can operate tube 855 alone. The purpose of. the'pulses coming over the T channel is to assure proper timing coordination of the transmission of signal pulses. In some instances such added precaution will not be necessary in which casethe T channel may be omitted, including the chain of tubes H124 to I029 of Fig. 10 and the transformer 855. In this case also the adjustment of the transformer 852 alone and tube 855 is such that a pulse on 852 will then be sufficient to cause transmission.

. Receiver The problem of recovering the modulation function at the receiver station is somewhat Simpl'er and will be first described by means of the block diagram of Fig. 2. For the example here eam synchronizediwith the incoming, pulses; perhapse by;means-of= a marker pulsesuch asithetM-pulseo 116*? mitting-zunit- VILoLFig; 8.. This IZOGBIQHISQimw-F- sage? isi,-amp1ifieclz.to'- any necessary extent :-as:-: illustrated'bythetube I I08 and theoutputtheree offis showrnas-going:- toia pluralityof: controls from the transmitting;station;so thatzitsioperae devices: C'1i Cm one associated with eachnofri tions coordinataproperlyi-withathe incoming signals; This generator sends 0l1t=p111Ss':tO'Va1i0l1S- devicesiwhich also receivethe signal: ThBSGQTGrxceived signals alone, or :the "pulses alone: will note a? plurality of attenuators in a manner hereim-iv after'to be described.

Receiver pulse generator VIII cause-'operati'onh A: signalzpulse inconjun'ction lo Aderived path'flom the Output Offl fi fi' with a pulse from VIII will cause a devicecton operate;

At'the time zwhenz-the signal corresponding; to" polarity app-ears the pulse generator appli'essaz:

throughsuitable amplifiers-as shown at I I.I2 and-.1 II'Ilhand an outward pulse therefrom isx-used tor;- control a relaxation oscillator-shownin:.Fig.,13a This I relaxation oscillator, centering about the signal i. e." no signalpulse the 'phasershifterrisis set; tbfshlft? phasei180: degrees ifz'ianzfonffsignail? the? phase shifter is set =.to shiftphase? zero; de=-: grees The phaser'shifteri-emains in: this'positibnn untillreceivinganother P pulse;

The Y next pulse of the pulse generatonzisi-senir to; the control Cr which controls -a:.change1to *at-b tenuator A1 proportional to"thatof-+Ai;:offFig; 11 The attenuation-is initially at..a minimum. If the received signal is an off signal, attenuation islets at a"'minimum; If' *an=on' signal"is received thi's causes operation'and A 1 is switched-"in The *nextpul'se from VIII'goesto Ci'at' tlie same timethat th'e signal 'from A2" of Fig: 1 arrives-=- and 'so onlthrougli' Am Thus, thertotal attenua-e ti'on is'- made-'the complement to that' introduced" 'inzthetransmitter of Fig. 1 which gave-rise t'oith'eon and oif signal's. Then after-a short time; perhaps the 'neXt marker pulse corresponding-136* time -tm+1 at'the'sender, apulsef'rom th'e'pulse generator enables the local oscillatonXi" This acts'as :anconstant' voltage source of desiredfre quen'cy' impressing'a voltage on the input of "the attenuator and sending" current through resist an'ce R and to the-homodyne detector XI; R' made-large in order to'add negligible attenuation: Frfom'the'voltage drop across'R there isproduced an "output pulse i from the 'homodyne detector XI nearly proportional to the amplitude M at the sampling time. A reset puls'emay follow the pulseto' oscillator X; This-will reset'Ai An and" phase shifter 'IXto the initial position in-prepara--- tion for succeeding samplecodes; The pulses" from the detector XI are passed'through' a low== pass filterXII. If the" highest frequency fm in theicomplex Wave is f1 l= T and 'if the low=pass= filter has a cut-off of fin; a signal 'proportionalto M, but'delayed perhapsby Tseconds; isrrecov ered at" the output 'of the low-pass'filter: Here T" is? the length of a" period"b'etween" tm --and"'tsi+i; i. e: between sampling-times."

With this brief description of the'blockfiiagram" off Fig: 2' we" may novwproceed' to a more 'deta'iledi description of devices"whichwillaccomplishthe" steps set forth above. It will be apparent to those"- skilled in the art that 'there are numerous circuit arrangements for accomplishing this. Certain specific arrangementsare here shown in Figs: 11, 7 l2 and 13 buttheseareillustrative for-the' sake of concreteness and iti to be" understood tliat;

manyvariations may-be made.Withoutdeparting fr'omthe spirit of my invention;

Referring more specifically toFig. 11, there is: shown a receiving unit H06; here indicated" as a radio receiver associated-swith a suitable receiving antenna H55. This unit H96 is a radio receiver of-any suitable type includingiafdetecton. theoutput' of' which; yields the pulse; signals? as:

areproduction of thepulses varrivingat'thetran'sa -The. adjustment of.- the parameters in the-relaxae tiona'foscillator of Fig. 13, however, is such-that;

the: circuit. does not normally oscillate: butIis;

triggered by a -Dulsewarriving from tube- II;I4=:. Furthermore; the parameters of this relaxations,

;;oscillatorrare so: adiustedathat the circuit-Milli;

be -triggered by. the first pulse in-a group (corre--- spending to the M pulse at thetransmitter): afterr whichthe oscillator -cannot betriggered until their arrival of the next M pulse.-

This isaccomplished by theuse of the'nlong initial. T; pulseaof- Fig-.7 4i-to whiclrtreference:has been-imade: It. is to -be borne int-mind that. a-ll'li pulses-transmitted 1by1VII areeoi the same amplie tuden However, sincertheitube II I2 is essentially;-

{at constant. current-device, the voltagevbuiltiup in the tank circuit I I:I3'-is proportional to :the-: duration'ofthe. incoming; pulse and thus ftheslong initial T pulses arriving at: I3 I I'Jv and corresponds: ing'ato: the M pulses will be of greater;. perhapsi double; amplitude-and so:be able-to triggertt-her relaxation oscillator; whereas? the other inathe cycle will not.

In a, manner analogous to-that of Fig; lfi'theree is associated with the relaxation oscillator; a

pulses timestick I3I6 from which a series ofv pulses.v

may be derived with a'time spacing as nearly: identical as may be necessary to the time spacing" of the pulses derived-from the tlmesti'cki atithe'r transmitting; station'sv This: timestick': has anv additional section: givingrrise 'to' a pulse indie cated by n and delayed only slightly behind the previous pulse. The function of the pulse n will be given hereinafter. The timestick is terminated with a suitable impedance I3 I 8 'to sum-- vpressrefiection. Also there is a series'oftubes= I325 to I329 from which a' series of positive-pulses derived from cathode followers is initiated, corresponding to the pulses from the timestick.

Receiver attenuators XII] The utilization of the various pulses to control the setting up of a series of.attenuat0rs A1 As in -cons'onance' with=-those set up at the transmitting station Will now be described. For this reference may be made to Fig. 12 and the right-hand portion ofiFig'. 11. In this latter portion there is shown a seriesof tubesrII45,-. I I65and II 85iconnected in tandem and; analo-- gous to thecorresponding circuit voflliigh 8.. They coupling from One tube to the next is a resistance capacitance network, a resistance therein being subject to change; wherebwthe. portion of the voltage generated in one. tub e: and passed-,toi-the next. is attenuated by; definite; amountszs, the: amountiofithlsiattenuation being controllable lm accordance with the code signal which-is-being received. Thus, the output circuit of tube II45 includes the attenuator circuit A1. A1 comprises the resistances R1 and R1 connected in series, the intermediate point being connected to the three stage amplifier comprising the tubes I24I, I242, I243 with resistance capacitance or other suitable coupling, there being a feedback connection from the plate of the last tube I243 through condenser I244 to the grid of 'I24I.

There is either zero or a large gain around the loop depending on whether the control voltage appliedto terminals. a and b cuts off one.v or more tubes in the loop or allows them to operate.

One mode of operation will be described in connection with A1. Here,.as in Fig. 9, the loop is normally held open by a sufficiently negative bias on the grid of tube I242. Under these. conditions, the grid resistance in the input of tube I I65 is R1+R1. If a positive pulse of suificient magnitude arrives at b then the loop is closed and if the gain through the circuit is high, the grid resistance is essentially R1, almost independent of gain, and a corresponding attenuation is introduced. Again, whereas for illustrative purposes the control has been shown on one tube only it may be desirable to control the bias on several or all tubes in order to open the loop completely.

The control portion C1 of the attenuator may take on a large variety of circuit forms so long as it performs the desired function in response to the pulses reaching it and may comprise one or more diodes or combinations of diodes and triodes or multigrid tubes in a variety of ways as will be clear to those skilled in the art. Specifically in Fig. 12 it is shown as a combination of a diode and a triode. A positive pulse correspending to the M pulse from the timestick operates through transformer I252 so poled as to make the cathode of diode I25I negative, whereupon any positive charge on condenser I254 is discharged and the negative bias on tube I242 opens the attenuatorloop. This occurs simultaneously on all of the attenuator units at the beginning of a cycle and sets all the attenuators at minimum attenuation.

In due course a pulse from circuit I of the timestick and tube I321 arrives at transformer I256 being so poled as to make the grid of tube I25'i positive. However, the plate circuitof. I251, which includes the transformer I258, includes a battery I259 with the negative terminal toward the plate so that the positive potential on the grid is not able alone to produce a current through the plate circuit and therefore no charge is placed on condenser I254. This is the condition which exists in case of an off pulse in which event no corresponding pulse passes through the primary of transformer I258. It will be recalled that the off signal corresponded to the introduction at the transmitter of attenuator A1 and it is noted that corresponding thereto such attenuation is not introduced at the receiver. In case of an on signal there will be a pulse in the secondary of transformer I258 which is so poled as to give a positive potential to the plate I251. This pulse alone is not sufficient to cause the current to flow through the tube because of bias on the grid. If however, there is received simultaneously a pulse from the pulse generator rendering the grid positive then there will be a flow of current charging the upper plate of condenser I254 positively and so closing the loop of attenuator A1 and introducing the corresponding attenuation. The charge on condenser I254 will persist until the time of the next M pulse whereon the condenser will be discharged as heretofore noted.

Thus it is seen that attenuator A1 will. not be introduced if -thecorresponding attenuator A1 hasbeen introduced at the transmittingstation. If it has not been introduced at the transmitting station, it will be introduced and remain in at the receiving end., Precisely the same operation will take. place for each attenuator A1 An after which the combination of attenuators connected in the circuit will be the complement of thatat the transmitting end. Thus, whatever attenuation is introduced at .the transmitter it will be omitted, at the receiver and the reverse.

I Local oscillator X The receiving station. is provided with alocal 20 oscillator X shown in Fig. 11 as II20. This may, but need not be of the same frequency as the local oscillator 11 at the transmitting station. In other words no synchronism between the two oscillators is required.

Phase shifter IX The phase shifter. IX is shown as comprising two triodes I22I and I222 the grid circuits of which are supplied" in' parallel'from the'local d oscillator H20 through the transformer I223. The output circuit'comprises transformer I224 the mid-point of the primary of which is connected to the positive the battery. The grid circuit of tube I22I 'contains the condenser I225 and the gridcircuit of tube I222 includes the battery I226 which tends to give a positive bias to the grid. Normally; therefore, the transconductance of tube I222 will be higher than that of I22I and there willbe an alternating current 40 of local oscillator frequency in the secondary of I224 of one phase. The phase of the'curren't in the secondary current may however be reversed by means of the phase shifter.

This phase shifter comprises two diodes I23I and I232 biased so that normally they are nonconducting. When a pulse from the tube I326, corresponding to P pulse, arrives at transformer I233 it is so poled as to render diode I23I conducting, giving a positive charge to condenser I225 of such magnitude as to give tube I22I a higher transconductance than tube I222, whereupon the current in the secondary of I224 is reversed in phase. This reversal occurs if the polarity pulse of the code at the transmitter was an off signal, meaning an absence of 'a received pulse. The condenser I225 is so connected as to retain its charge for the duration of one complete cycle.

If, the polarity of the sample of the complex 50 wave at the transmitter had been negative then an on P polarity pulse will have been transmitted and, in turn, received at the receiving station. A corresponding pulse, therefore, arrives at the transformer I234 in parallel with the [$5 transformers I258, etc., at each of the attenuation control circuits. The transformer I234 is so poled that its pulse opposes that of the pulse coming on transformer i233 and consequently diode I23I does not conduct, the condenser I225 does not become charged and the phase of the oscillations in secondary of I224 is not reversed. At the end of the cycle, or the beginning of the next cycle, the M pulse from the tube I325 operates through transformer I236 to make diode I232 conducting whereupon-the condenser I225 amazes is; discharged, or; freset, to; normal. condition. 'llbr ushztne means. thus. d scribed, it. is. seen tha 1.1 s possi le to chan e. the. phase of the ca oscillator currentin. transformer I224. by 18o. degrees.

'liheloutput, of. i224. of, local oscillator frequency isdmpressed, on the. inputv circuit. I mo. of. the first attenuator. tune i145. In the outputclrcult. of thelastattenuator tube. llilitnere is included the resistance linev corresponding to. R. of Fig. 2. illns. resrstanceis. large compared. to the resistance-Rn. of the last attenuator and. consequently adds noappreciable-attenuation. Thus it 1888611 thatthe amplitude ofthe currentin m 6 and the potentiai' VZLI'laBIOIIS across its terminals, as well as the current in-the transformer I21, will be proportional to. the sum of the attenuations which have not been introduced at the receiver. Consequently, they will be proportional to the sum of the attenuationswhich were introduced at the transmitter. and-therefore proportional to the amplitude of the complex wave sample. Furthermore; its phase will bedetermined by phase shifter IX to correspond with the polarity ofthe complex wave sample.

Homodync detector-XI The homodyne detector XI includes a balanced demodulator comprising the varistors IZII and I212 connected in a standard bridge circuit. This demodulator is supplieddirectly with, local oscillator frequency through transformer I213 and also with the same Irequency through tube I215 and. transformer IZM. There will then appear over the resistor l2l'l potential difierences of sampling; frequency, each. element being proportionarin. every respect to the sampling cur- 'r.ent ,or voltage at the transmitter. If the polar,- ity ofthesampling current at the transmitterre- 'verses, then thev voltage, across the resistor I2l'l will, also reverse. I The last step. of interpretation may come through the multigrid tube I218. This, tube is normally biased. to cut-off. However, through the additional section of .the timestick leading to the, pulse 11!, previously referred to, a positive pulselis impressed on the triodesl35l. and I352 connected in series to give requisite. amplification without, change of. polarity of the pulse. Very shortly after the termination of theamplitude code there. arrives thenon onev grid of tube l.2|8 a; positive pulseof suificient value to.enable the tube lZHL-for the duration of the-pulse n. During this, interval then there appears in theoutput circuitof. I2l8 a pulse. determined by the magnitude and polarity ofthe voltageover l2l6, which latter is proportional to the amplitude. and polarity of the complex wave sample.

These pulses will arrive insuccession, one for eachsample from the complexwaye. By means of, thelow-pass filter l2l9 the undesired. high frequency, components may beremoved and, the resulting wave, passing to terminal equipmentand receiver, will be a reproduction of high fidelity of the original complex wave at thetransmitting station.

. During the setting up of the attenuations, pulsesmodulated ona suitable carrier will have beentransmitted from VII- bearing the informat-ionto the remote station, on what attenuations. are being introduced. The amplitudeofeach of the pulses so transmitted from VII. will bethe same, and eachelement of. the signal is purely an foif and on matter. Since only integers are; sentsucn a. si nal can be. repeated; without added oistortionor noiseto the recovered, 111116111? gene-e even though distormon. ornoise below a certain thresholdlevel may be present. in the repeaters. 'Ihus,\,evenfor very high quality trans misslon the requirements; on. the. repeaters: are very low. Thismaxes possible transmission over long paths with, frequent. repeating. The. presence or. introduction of noise in; the transmission path from. VII to the remote. receiving" station willhavena influence so long as the. noise introduced. is. relatively small compared .with the signal: being. transmitted: over the path. Such noise therefore willnot appear in the signallater reproduced;

This application is related to. my'copending application Sen. No. 603,989, filed July 9, 1945', Patent-2,508,622 of May 28; I950, and. differs from it in the. manner of building up attenuation. Whereas in that application the conductance steps are additive on a unit or multiunit basis, in. this application the changes are on a decibel basis, that is, the attenuations herein are in suceessive steps of multiples. This has certainadvantages one of which is the much wider range of amplitude which can be covered by a code of a given-number of units. Thus, inmy aboveidentified application fora code of n digits the range of selection of amplitude in voltage (or current) is 2 whereas in this sytem the range of selection of amplitude in decibels is 2 If 11.27 and if the reference level is taken as ea (perhaps in millivolts), thenfor the copending application therange of amplitude'is from co to 2 60- (:128 60', equivalent to, 42 decibels).. This meansa granularity of coin a maximum; contemplated signal'of" 12860; that is, an accuracy Within 1 per cent. However; the granularity is still eo no matter how small the input, and so for-an input as low as 280 the accuracy is about 50 percent. In-the system of this application the percentage accuracy of reproductionwill be the same without regard tov the amplitude of the output. Thus, if the smallest step is .5Ldecibel, thenthe error: in: determining the value ofinput voltage will not be greater. than. 6jper: cent. and will average about half" of this. Furthermore, for a seven-digit system the range. inamplitude willbe from (thatis about 1 to 1500).. This feature of a non-linear scale (wherethaelectrical quantity builtup is not proportional. to -the.voltage:or cur.- rent amplitude-of the sample) isequivalent to a typeof volume compressionv in:the cod-ed signal andthis isan essentialaspect of thisiinvention. The particularnon-linearity.which has-been used for il ustrationis a decibel scale (theelectrical quantitybuilt: up is proportional'to the logarithm of: the voltage or current amplitude of the sample) I and this, as pointediout above, not: only gives wider range but givessmaller granularity forlow amplitude samples.

In. considering the above system it will be apparent tov thoseskilled in the art that many variations: may be made in the-system thus-far described. without" departing from: the: spirit of the invention; For-example, the .attenuationscheme, which constitutes an important: element. in my invention, may-be; materially alterednas indicated in Fig. 1.4. Therejthe plurality:ofresistance.com-

- binations..-:Rr.and R15, R2 and-Re, etcis shown 21 as a series of potentiometers connected in tandem. The portion R1, R2 Rn of each potentiometer is capable of virtual short circuit as already indicated. The portion R1, R2, etc., of the resistances will have to be large compared with the electrically controlled shunt portions if the attenuations caused in the various potentiometers are to be substantially independent. This may be somewhat difficult to attain and for this reason isolation of the electrically controlled resistances by means of vacuum tubes, as already described in detail, may be desirable.

Other changes may be permissible, some of these having been already mentioned. For example, the T channel of pulses maybe omitted with corresponding simplifications although with some. loss of control. Also, the local oscillator at the receiving station may be on all the time instead of being triggered on occasionally, for even if on, it is inefiective at the terminal apparatus unless and until the tube l2l8 has been enabled by the pulse coming from n supply. Still further, it will be evident that modulation and demodulation ieatures of the transmitter station maybe omitted, the sample signals of the tube 826 going directly to the input of tube 835 and directly from tube 9M to the resistor 93?, or its equivalent, Without the intermediation of the demodulator in the homodyne unit IV. This would mean also the omission of the local oscillator. Corresponding alterations could be made at the receiving station in connection with its local oscillator. In general, however, such omissions or simplifications will lead to sacrifice in operation or quality and it will be a matter of engineering judgment as to how far one may carry out such simplifications.

. What is claimed is:

1. The method of transmitting information on the shape of a complex signal wave which comprises taking amplitude samples of the Wave at equally spaced intervals, building up an attenuation network for each sample step by step to a magnitude proportional to the sample amplitude and transmitting oil or on pulses in accordance with each step, the steps being related to each other on a non-linear basis.

v 2. The method oftransmitting information on the shape of a complex signal wave which comprises taking amplitude samples of the wave at equally spaced intervals building up an attenuation network for each sample step by step to a magnitude proportional to the sample amplitude and transmitting off or on pulses in accordance with each step, the steps being relatedto each other on a logarithmic basis, the total attenuation on said basis being equal tothe sum of the attenuations for the individual steps introduced. I v

3. The method of claim 1 with the added step of introducing at a receiver an analogousattenuation network, the total steps of attenuation there introduced being the complement of those introduced at the transmitting station.

LA system for transmitting information on the shape of a signal wave comprising a circuit for periodically sampling the amplitude of the wave,'means for building up an attenuation network by a series of non-linearly related steps to a value proportional to each sample, means for generating a cycle of pulses for each sampling operation, means responsive to the first pulse of the cycle for controlling the sampling circuit and means responsive to succeeding pulses of the cycle for controlling said means for building up an electrical quantity in coordination with the respective sample.

5. A system for transmitting information o the shape of a signal from a transmitting to a receiving station, the transmitting station com-. prising means for sampling a signal wave periode ically and storing on a condenser a potential proportional to the sample amplitude, an amplifier tube the input of which is connected across the storage condenser, means for connecting in the circuit of said tube a multiple attenuator comprising a plurality of attenuators in tandem to attenuate the effective output voltage, a pulse generator means controlled to build up the said attenuation step by step by the coordination of the signal amplitude with pulses from the pulse generator until the residual effective output falls to an arbitrary low reference level.

6. A system for transmitting information on the shape of a signal from a transmitting to a receiving station, the transmitting station comprising means for sampling a signal wave periodically and storing on a condenser a potential proportional to the sample amplitude, at constant current source producing a current proportional to the charge on the storage condenser for'the duration of the sampling period, a train of unilateral attenuating devicesoperating to attenuate the current, a control circuit for each attenuator, each attenuator adapted to introduce or not introduce its attenuation subject to the control circuits, the control circuits being operated successively by pulses from a pulse generator and introducing attenuation if the residual of the sample amplitude is in excess of an arbie trary small amount.

7. The combination of claim 6 characterized by the fact that the train of unilateral attenuating devices comprises a series of amplifier tubes connected in tandem, the coupling circuit from the output of one tube to the input of the next including a coupling resistance a portion of which may be substantially 'short-circuited to introduce a designated amount of attenuation, the introduction of said attenuation being subject to the control circuits.

8. The combination of claim Bcharaoterized by the fact that the train of unilateral attenuating devices comprises a series of amplifier tubes connected in tandem, the coupling circuit from the output of one tube to the input of the next including a coupling resistance a portion of which may be substantially short-circuited to introduce a designated amount of attenuation, the introduction of said attenuation being subject to the control circuits in such manner that if the residual of the sample amplitude at any stage is in excess of an arbitrary small amount, the next step of attenuation is introduced and it below said arbitrary amount, it will be first introduced and then removed.

9. A system for transmitting information on the shape of a signal wave comprising a circuit for periodically sampling the amplitude of the wave, means for building up an electrical quantity by a series of steps related on a logarithmic basis to a value proportional to the signal amplitude, means for testing the polarity of the sample, and means for generating for each sampling operation a cycle of pulses, means responsive to the first pulse of said cycle for controlling the sampling circuit, means responsive to the second pulse of said cycle for controlling the means for t n th rq r wg he sampl d .me nsrea amazes sponsiye? to succeeding pulses of. saidi cycle: in coordination with the sample for controlling. said means forv building. up an. electrical: quantity.

10. Apparatus. for transmitting a complex wave form comprisingtmeansfor settingv upa series of groups of: permutativelycoded signaling. pulses; the. series. representing: a succession of instantaneous amplitude samples of. the wave: form. and eachpulse of a group representing a different fraction of the amplitude ofa wave form. sample; the: means therefor: comprising a plurality of. attenuators adapted to be connected in tandem, each. element of attenuationbeing proportional to an fractionv of l the corresponding, sample; amplitude and the surn of" the attenuations: introduced beingproportional t'o theinstantaneousamplitude off: the wave form;

11. Apparatus for reconstructing a complex:

Wave= formifrom a. series of groups; of. permutatively coded signaling. pulses, .the series repre' senting. a. succession. of. instantaneous. amplitude samples oi" the. complex wave. form each pulse: of a: group representing a different fraction. or the amplitude oflsaid wave form sample. the: means therefor comprising. a plurality of attenuators to be introduced: in tandem; the totalattenuation being. proportional. to theisum of.-: the. individual attenuations, thissum subtracted from .a: constant being. proportional. to the instantaneous amplitude I f the complex. wave sample.

12.. A system for? transmitting. information on the shape of: aisignal Wave from a. transmitter to a: receiver station, the transmitter station comprising av circuit for periodically} sampling. the amplitude. ofthe wave, acurrentsource; adapted to deliver a current proportional: to: thesample amplitude; and constant for the duration of a sampling interval a load supplied with current from.saidsource, a circuit for. transferring the effective voltage acrossthe. load to a polarity and amplitude detecting circuit,-a plurality of n attenuators to be connected intandem in the lcadcircuit of said current generator and thus reducing. the. voltage reaching the polarity and amplitudedetector, acontrol. circuitfor each-attenuator; a pulse generator adaptedto generate cycles. of pulses,.one pulsein-the cycle serving. as amarker pulse and as. timing. the sampling, of the. signal wave,v another serving as. a. polarity pulsei'tc cooperate.intesting the polarity of the sample amplitude',.the remaining pulses. operating, successively through the control circuits of the. attenuators in cooperation with the residual amplitude reachin the amplifier detector to introduce one or more of the attenuators andthus to reduce the residual reaching the amplitude detector to an arbitrary small value, a circuit associated'with each control circuit to transmit an off signal ifiits attenuator isleftin and. an "on signal if it'isremoved, the n. attenuators being graded in size from'the'smallest of value A; the largest being first tested for introduction, the systemso operating that the total attenuation on a' decibel basisintro'duced' and left'in' at the end of the cycle is proportional to the sample amplitude.

13; A- system for transmittinginformation on the shape of a signal wave from a transmitting, to arecei'ving' station, the transmitting station comprising a circuit for periodically sampling, the amplitude of the wave and storing a charge on a condenser proportional to the amplitude, a constant current source controlled by the*condenser charge and adapted to deliver a current proportional to the" condenser charge and" constant for the duration of a sampling interval, a: circuit for' transferring; the voltage across the load to. a polarity and amplitude. detecting, circuit,. a plurality of n attenuators to be-connected: in tandem in the load circuit of thecurrent generator to-reduce. the voltage reaching the polarity and amplitude detector, a control circuit for each attenuator, a pulse generator adapted to generatecycles of pulses each pulse consistingof 2+n. equally spaced pulses the first pulse in the cycle serving as a marker pulse and as timing. the sampling of the signal Wave, the second serving as a polarity pulse for timing the operation of the-detector to test the polarity of the condenser charge, the next pulse operating through the: control circuit of the first and largest at tenuator withth'e signal amplitude reaching: the amplitude: detector to introduce the said firstattenuation and to leave it in if the residual then reaching. the amplitude detector is above an arbitrary'smallvalue and next to remove it if. the? residual is less than this small value; acircuit associated with the control circuit to transmit an-o signal if the attenuator is left in andan on signal if it is removed, each succeeding pulse in coordination with the residual then reaching the amplitude detector operating. inturn in the same manner through the controlcircuit to introduce, and then remove if neces sary,. its attenuator and to transmit corresponding signals, the n attenuators being graded in sizefrom the smallest of value A0 to. the largest of value. 2 A0 the total attenuation introduced and left in at the end of a cycle being.propor-' tional onadecibel basis to the sample amplitude, the size of the steps increasingin accordancewith: a binary counting system.

14. In a communication system, apparatus for sampling a signaling wave at regular occurring instants of time, a source. of high frequency alternating' current, means for maintaining the magnitude of said alternating current between said instants of time at a value determined bythe'magnitude'of said signaling wave at thelast' instantofthe sample, a source of reference voltage, apparatus for: comparing. a fraction of said:

alternating current v with said reference voltage, equipment-for sending either one of two signaling-:conditions, and apparatusresponsive' to said comparison apparatus'for controlling which of said two signaling conditions are transmitted incident to said'comparison.

l5. Ina communication system, apparatus for sampling a signaling wave at regular occurring instants of' time, a source'ofhi'gh frequency a1- ternating current, means for maintaining the magnitudeof said alternating current between saidinstants of time at a value determined by the magnitude of said signaling wave at the last instant of the sample, a source of reference voltage, apparatus for comparing a fraction of said' alternating current with'said reference voltage, equipment for sending either one of two'signaling conditions and' apparatus responsive to said comparison apparatus for controlling which of said two signaling" conditions are transmitted incident' to said comparison, and equipment for thereafter" changing the fraction of said alternating' current and repeating theprocess of comparison and'signal transmission.

16. A signal transmission system comprising a source of electrical current, asource of reference voltage, switching: equipment for comparing a fraction of'sa'id current with said reference voltage, and signal transmitting equipment for 

